The majority of weather radar systems in operation today utilize a single polarization strategy to enhance precipitation reflectivity. Liquid hydrometeors (e.g. raindrops) deviate from a sphere when their radius is greater than about 1 mm and have a shape more like that of an oblate spheroid with a flattened base (similar to a hamburger bun) that gives a slightly stronger horizontal return. Hence, current radar systems are typically horizontally polarized to enhance precipitation returns.
However, singly polarized radar systems have severe limitations in regions with partial beam blockage and such systems do not facilitate hydrometeor classification. To overcome these shortcomings of singly polarized weather radar systems, systems with alternating pulses of horizontally and vertically polarized signals have been developed. These dual polarized radar systems, sometimes referred to as “polarimetric weather radars,” offer several advantages over conventional radars in estimating precipitation types and amounts. Foremost among these advantages are the capability to discriminate between hail and rain, detect mixed phase precipitation, and estimate rainfall volume.
Current dual polarized radar systems utilize polarization that is altered sequentially between linear vertical and linear horizontal to capture data enhancing values, such as, for example: (1) reflectivity factors at both horizontal and vertical polarization; (2) differential reflectivity for two reflectivity factors; (3) cumulative differential phasing between the horizontally and vertically polarized echoes; (4) correlation coefficients between vertically and horizontally polarized echoes; and (5) linear depolarization ratios. In addition, Doppler velocity and spectrum width can be obtained by suitably processing the horizontally and vertically polarized return signals.
Dual polarized radar systems also allow for the implementation of precipitation classification schemes from inference radar processing of hydrometeor shapes as discussed in various papers authored by practitioners who work in these areas, such as, Ryzhkov, Liu, Vivekanandan, and Zrnic. In addition, by looking at phase differences between the horizontal and vertical components, the effects of partial beam blockage can be mitigated and greater clutter rejection can be obtained.
Many current dual polarization weather radar systems locate the radar system processor above the elevation and rotary coupler and above the requisite pedestal mounting 16 with feed horns and antenna as shown in representative system 10 at FIG. 1.
Zrnic disclosed a theoretical simultaneous dual polarized radar system by replacing the orthomode switch with a power splitter and an orthomode coupling at the antenna feed horn in U.S. Pat. No. 5,500,646. Zrnic also worked out the various calculations pertaining to simultaneous dual polarization radar systems as recorded in the '646 patent, not already known in the industry, and such calculations are hereby incorporated by reference for background information into this disclosure and are applicable for the presently disclosed systems. While Zrnic displays a theoretic simultaneous dual radar system, a number of systems exist today for the actual capturing of radar reflectivity data and incorporation into local workstations or a nation-wide network of radar installations. One such system is disclosed in a white paper authored by A. Zahrai and D. Zrnic entitled Implementation of Polarimetric Capability for the WSR-88D (NEXRAD) Radar, published in American Meteorological Society in 1997 in section 9.5, which is hereby incorporated by reference. Additional comments pertaining to the capturing of reflectivity data and the processing of such data will not be made as these incorporated references describe the basic theory and operation of such systems and such information is already understood in the industry and not necessary for a complete understanding of the herein described invention. Dual polarization weather radar systems are disclosed by Alford in U.S. Pat. Nos. 6,803,875, 6,859,163 and 7,049,997, which are incorporated by reference for background information in this disclosure.
Practical problems exist in the current dual polarization weather radar systems and in the above referenced Alford models. Locating the digital receiver and processor below the elevation rotary coupler 34 in the radar pedestal increases the number of necessary waveguide channels traveling through the rotational coupler. Dual channel rotary couplers are expensive and they also introduce phase errors between channels that vary with rotation. Such errors require complex compensation processing in the received radar returns limiting the reliability of reflectivity data in simultaneous dual polarization weather radar systems. Locating the signal processor below the elevation rotary coupler 34 creates problems in data transmission from the receivers to the signal processors. First, a complex and expensive slip ring would be required to transmit the data. Second, streaming raw I/Q data digitally over long distances introduces errors into the data resulting from noise introduction in the transmission cables. Due to the relatively large data bandwidth required in a dual simultaneously polarized radar system, differential parallel transmission lines must be utilized and data integrity can be impacted in electrically noisy environments. Providing an alternate path for data transmission from the receivers to the signal processors would simplify the slip ring assembly by requiring only paths for power, reference, Ethernet communications, and antenna motor drives.
In addition, the “receiver over elevation” configuration 10 such as is shown in FIG. 1, locates active electronics, such as the receiver and signal processor, in an enclosure in the pedestal. The enclosure must be environmentally controlled with, for example, solid state heating and cooling units. Components may fail under high thermal loads and a simultaneous dual polarization radar system may experience changes in performance as temperature fluctuates. Maintaining a known environment can be costly and difficult, especially when the required known environment is located high up, above the elevation rotary coupler 34 in the radar pedestal.
Therefore, what is needed is an improvement in dual polarization weather radar systems from the current methods of locating the signal receiver and/or the signal processor above the elevation rotary coupler 34, or locating both the signal receiver and the signal processor below the elevation rotary coupler 34. An alternate data path for data transmission from the receivers to the signal processors is needed.